While most of us take soils for granted, researchers from KAUST’s Energy Geo-Engineering Laboratory literally know soils inside out. Their research has important implications for diverse geotechnical projects, including mining, oil extraction, bridge and tower foundations, coastal and offshore structures, analyzing tunnels and sinkholes, and designing earthquake-resilient infrastructure.

Professor J. Carlos Santamarina, postdoc Junbong Jang (now at the United States Geological Survey) and PhD student Junghee Park have designed a new classification system for the world’s soils that will enable engineers to predict more accurately their properties and behavior based on simple metrics.

At present, most of the world’s geotechnical studies use the Unified Soil Classification System (USCS), which has its roots in the construction of World War II airfields. Since then, it has been refined, but not fully updated. Meanwhile, soil data have accumulated extensively and, Santamarina explains, “led to a deeper understanding of sediment properties and behavior,” suggesting the need for a thorough re-evaluation of the system.

A new state of matter

We tend to consider matter to be solid, liquid or gas. However, particulate materials, such as soils, may act differently to any of these states. Santamarina describes soils, enigmatically, as “inherently nonlinear, nonelastic, porous, pervious and effectively stress dependent.”

The USCS defines soil types by particle size and plasticity, or how they deform when mixed with water. However, soils can comprise complex mixtures of differently sized particles, and furthermore spaces between particles are not empty but filled with liquid, gas or both. Santamarina enthuses, “The coexistence of these materials gives rise to fascinating emergent phenomena and puzzling responses, such as liquefaction.”

The new revised soil classification system1,2 takes into account not only grain size but shape and provides a more accurate representation of the transition zones between soil types. It places more importance on the role of the smallest soil particles, known as fines, and the chemistry of the surrounding fluid, which influences many geotechnical phenomena. Crucially, the new classification system enables engineers to distinguish the soil fraction that is primarily responsible for carrying any weight placed upon them and the soil fraction that controls the flow of fluid.

Soil classification is a relatively simple process of determining a set of parameters using bench-top devices readily available in soil laboratories worldwide. These can then be entered into the equations detailed in the KAUST team’s publications, or analyzed using a spreadsheet or cell phone app, both available on their website. The result, for any given soil, is a two-part description incorporating both mechanical and fluid-flow properties; for example, ‘S(F)’ signifies a soil with mechanical properties controlled by sand, but with permeability determined by its fines component.

An ongoing endeavor

Response to the publication of the revised classification has been encouraging and collaborative. When the team published the first part, covering fines, “researchers from around the world reacted and contributed exceptional data to strengthen the classification,” says Santamarina. “We expect a similar response to the second part.”

He continues, “The reality of soils is more complex than the idealized systems created in the lab or on a computer.” A complementary soil database built at KAUST enables users to make robust estimates of the hydromechanical properties of natural soils. In fact, the researcher in Santamarina’s laboratory is geared toward a continuously evolving set of engineering analysis and design tools. The classification, says Santamarina, is just “a first step toward an integrated laboratory-database-IT system being developed with collaborators worldwide.”

No classification can be exhaustive: soils made of uncommon grains, such as diatoms or fly ash, will always present special challenges that engineers need to be aware. However, the classification aims to be robust enough to be used successfully even by those without field experience.

The team is currently working on a publication that will extend the application and interpretation of the classification in practice.

This multidisciplinary work—drawing on concepts from geology, physics and chemistry and oriented at disciplines from civil to environmental and energy geo-engineering—benefits from what Santamarina calls the exceptional research environment and expertise available at KAUST to generate tools with applications much further afield.